The invention relates to capacitive position sensors, more particularly the invention relates to capacitive position sensors for detecting the position of an object around a curved path.
Capacitive position sensors are applicable to human interfaces as well as material displacement sensing in conjunction with controls and appliances, mechanisms and machinery, and computing.
Capacitive position sensors in general have recently become increasingly common and accepted in human interfaces and for machine control. In the field of home appliances for example, it is now quite common to find capacitive touch controls operable through glass or plastic panels. These sensors are increasingly typified by U.S. Pat. No. 6,452,514 which describes a matrix sensor approach employing charge-transfer principles.
Due to increasing market demand for capacitive touch controls, there is an increased need for lower cost-per-function as well as greater flexibility in usage and configuration. Similarly, there is a significant demand for capacitive material displacement sensors (e.g. fluid level sensors, mechanical movement sensors, pressure sensors etc.) at lower price points, which cannot be easily met with current generations of non-mechanical transducers.
In many applications there is a need for a human interface having many keys or sensing positions, nearly (but not) akin to the flexibility afforded by 2-D touch screens or touch pads as typified by U.S. Pat. Nos. 4,476,463 and 5,305,017. For example, in a computer monitor, it is desirable to have controls on the screen bezel to allow adjustment of brightness and contrast; ideally, as in former times, a continuously adjustable control (e.g. a potentiometer) is used to control these parameters. Due to price pressures and aesthetic requirements, these have been usually eliminated in favor of just a few bezel-mounted menu selection buttons which are much harder to understand and adjust.
In the fields of electronic and medical test instruments, LCD displays are often used in conjunction with rows of bezel buttons to provide software-driven menu functions. Many such applications do not allow the expense, reduced contrast, and fragility of touch screens, yet meanwhile suffer from deep or limited menu options and visual parallax. An example of this type of menu control is to be found in almost any current cash dispenser such as the NCR LCD-5305. Manufacturers would use higher resolution controls on or closer to the edge of the screen if economic considerations could be met. Similar markets exist for domestic appliances, educational games, information/internet kiosks, and the like.
In the field of heating ventilation and air conditioning (HVAC), the state of the art in wall-mount thermostatic controls is currently exemplified by the Honeywell model CT8602, a menu-drive system with a small LCD screen. Advanced features in these devices are accessed via deep levels of menus which are often non-intuitive compared with simple dial or slider based controls.
Electromechanical human interface controls (such as pushbuttons, membrane switches, and potentiometers) have the noted deficits of being unreliable and subject to water ingress, as well as being only marginally compatible with LCD based menu systems. Classic user controls like dials and resistive potentiometers require panel openings which allow dirt and moisture to enter into the product. They also do not present a ‘clean’ appearance, are considered increasingly quaint, and seriously limit the flexibility of industrial designers. U.S. Pat. No.5,920,131 describes one solution to this problem, in the form of a rotary knob which is magnetically held to a seamless panel surface and which magnetically interacts with position sensing detectors below the panel surface. However this solution still requires a protruding knob and is expensive to manufacture.
More recently there has been the appearance of ‘scroll wheels’ as input devices, as typified by the Apple Computer iPod MP3 player, and shown in U.S. D472,245. These devices have either a mechanical input scroll device or a capacitive device based on circuitry from Synaptics, Inc. (San Jose, California, USA).
There exists a substantial demand for new human interface technologies which can, at the right price, overcome the technical deficits of electromechanical controls on the one hand, and the cost of touch screens or other exotica on the other.
In the field of mechanical displacement sensing, Linear Variable Differential Transformers (LVDTs) exemplified by the Schaevitz Sensors (Slough, UK) MP series exist to provide precision positioning information for feedback in process controls. Other smaller devices such as the Schaevitz Sensors XS-B series are incorporated into machines and instruments. Such devices are usually high-cost solutions albeit very accurate, and rely on magnetic field balance measurements made with expensive signal conditioners to operate. These devices exist to provide highly reliable non-contact sensing that can operate in harsh environments with great precision. They solve the wiper-reliability problem of resistive potentiometric methods by eliminating the use of a physical contact. Similarly, there exist capacitive position sensors as exemplified by the RDP Electrosense Inc. (Pottstown, Pennsylvania, USA) Rotational Capacitive Displacement Transducer (RCDT), which also requires a special, expensive signal conditioner to operate. An example of such technology is described more fully in U.S. Pat. No. 5,461,319 which describes a bridge based circuit. Capacitive based devices can accommodate both linear and rotational position sensing. For example U.S. Pat. No. 5,079,500 describes a linear or rotary ‘potentiometer’having a capacitive wiper, which makes for a highly reliable method of position sensing as it does not use a galvanic wiper. Adaptations are available to measure pressure and by inference, fluid level. The above referenced technologies however suffer the problem of being very complex and expensive to manufacture, limiting their use to high-end or industrial equipment. In addition, U.S. Pat. No. 5,079,500 makes use of an ‘active wiper’, that is, the wiper terminal is connected to amplifier electronics, making that invention unsuited use for human touch.
LVDT and RCDT type transducers work very well, but leave untapped a very large market for low cost devices which can be used commercially in automotive and appliance applications.
FIG. 1 schematically shows a resistive sensing element 2 for use in a linear capacitive position sensor described in the present inventor's U.S. Pat. No. 7,148,704, the disclosure of which is incorporated herein by reference. The resistive sensing element 2 extends between first and second terminals 4, 6. Each terminal 4, 6 is connected to a respective sensing channel 8, 10. Each one of the sensing channels is operable to generate an output signal which depends on the capacitance between the terminal to which it is connected and a ground 14. The resistive sensing element 2 has an inherent capacitance Cd to ground 14. This inherent capacitance Cd is distributed along the length of the resistive sensing element 2, as shown schematically in FIG. 1. When a user's finger 12 is located close to the resistive sensing element 2, the user provides an additional capacitive coupling between the resistive sensing element 2 and ground 14. This additional capacitive coupling is schematically shown by capacitor Cx in FIG. 1. The presence of additional capacitance Cx modifies the capacitance between the each of the terminals 4, 6 and the ground 14. In particular, it is found that the ratio of the changes in capacitance associated with each of the terminals depends on the position of the finger along the resistive sensing element. For example, if the ratio is unity, the finger is midway along the resistive sensing element. Using this type of position sensor, the position of a finger can be determined independent of the magnitude of the capacitive coupling provided by the finger. This means the sensor can be used with different kinds of pointer, e.g., fingers and styli, while providing consistent results. This scheme provides for a simple and effective alternative to more complex LDVT sensors.
In many cases a rotary position sensor, i.e. one which is capable of determining the position of an object along a circular path, is desired. For example, to provide an equivalent to a mechanical rotary knob which can be used in a touch sensitive control panel.
FIG. 2 schematically shows a plan view of a resistive sensing element 16 for use in a rotary capacitive position sensor of the kind proposed in U.S. Pat. No. 7,148,704. The general functions of this device are similar to the device of FIG. 1. Many features of FIG. 2 are similar to and will be understood from correspondingly numbered features of FIG. 1. However, the resistive sensing element 16 of FIG. 2 differs from that of FIG. 1 in that it is arranged in a curved path, namely an arc. The underlying operation of a rotary capacitive position sensor based on the resistive sensing element of FIG. 2 is as described above with respect to FIG. 1. However, because of the arcuate arrangement of the resistive sensing element, the position at which a finger approaches the resistive sensing element can be translated to an angular position about the center of the resistive sensing element. Although the resistive sensing element 16 of FIG. 2 can provide for a simple yet effective rotary capacitive position sensor, it is limited as to the angular range over which it can operate. For example, in FIG. 2 it can be seen that the terminals 4, 6 defining the extent of the sensitive area of the position sensor are separated by around 30 degrees. This provides for a sensor with a sensitive angular-span of around 330 degrees and a dead zone of around 30 degrees. The dead zone between the terminals cannot be made arbitrarily small. This is because a finger placed near one terminal would otherwise provide a direct capacitive coupling to ground for the other terminal due to its proximity thereto. This can lead to ambiguity in calculated position. For example, with a finger placed over one terminal which is located too close to another terminal, the respective sensing channels 8, 12 can measure broadly similar signals. Similar signals is also what is seen when a finger is placed about midway around the resistive sensing element and hence a reported can be ambiguous. It may be possible to remove the ambiguity based on the overall magnitude of the signals detected, but this requires a priori knowledge of the typical magnitude of capacitive coupling to be expected from the finger, or other pointer. This can mean that different magnitudes of capacitive coupling can lead to differently calculated positions.
A similar ambiguity arises whenever a finger can provide a direct capacitive coupling to multiple parts of a resistive sensing element, for example, where the overall scale of the resistive sensing element is small. In such cases it can be possible for a finger placed over one location on the sensing element to provide a direct capacitive coupling to ground for another location, even if the dead zone between the terminals itself is suitably large. This can again leads to ambiguity and so limits how tightly a resistive sensing element 16 of the type shown in FIG. 2 can be curved. This is a problem where space is at a premium, e.g. when designing control interfaces for small portable devices.